The present invention relates to a novel and improved chest drainage apparatus capable of accurately measuring gas leakage or flow from a patient's chest cavity, even at very low, near zero or zero flow rates.
Devices for measuring gas flow such as flow meters and hot wire annenometers are well-known and have been used to measure the flow of both gases and liquids in a wide variety of industrial applications. Examples of such applications included (a) controlling the amount of gas added to a liquid reaction component in the production of foam materials (see U.S. Pat. No. 4,050,896 to Raffel et al.), determining the respective flow rates of gas and crude oil in the diphase mixture during a production test of a well (U.S. Pat. No. 4,168,624 to Pischon), measuring the rate of flow of air from a tank to determine the volume of dry materials in the tank (U.S. Pat. No. 3,744,306 to Krueger), and measuring the flow rate of desolubilized gas from a liquid under vacuum to determine the amount of gas remaining in the liquid (U.S. Pat. No. 4,289,018 to Hellouin De Menibus).
However, while the utility of such devices has been proven in many industrial applications, they do not accurately measure very low or zero flow rates. Accuracy at near zero or zero flow rates is very often not critical or significantly advantageous for most industrial applications, but it is highly important in other applications such as monitoring leakage of poisonous gases from storage receptacles, as well as in certain medical applications such as the measurement of gas flow or leakage from a patient's chest cavity in a chest drainage unit.
A chest drainage unit is used to suction gases and liquids from the chest cavities, or pleural cavities of patients. The pleural cavity contains both lungs, which in their normal expanded state fill the pleural cavity. Several conditions and diseases such as emphysema and various infections can cause a build up of liquid and gases around the lungs inside the pleural cavity. This causes the lungs to collapse to a volume much less than that of the pleural cavity, severely impairing breathing functions. The lungs can be re-expanded to their normal state by draining the liquid and gases from the area outside the lungs using a chest drainage unit.
Chest drainage units are also used in the treatment of patients who have air leaks in their lungs, allowing excessive amounts of air to enter the pleural cavity. The excess pressure in the pleural cavity caused by the air leaks can be relieved through evacuation of the air via a chest drainage unit.
U.S. Pat. No. 4,372,336 issued to Cornell et al. and assigned to Sherwood Medical Industries, Inc., discloses and claims a conventional chest drainage unit similar to those contemplated for use in connection with the present invention.
In measuring the air flow from a patient's pleural cavity via a chest drainage unit, the physician is most interested in the measurement of air flows near and at zero. This is because patient air leakage progresses to zero in the final stages of healing, making the precise measurement of those final flow rates very important. There is therefore presently a great need for a safe, efficient, and accurate method of measuring air flow in a chest drainage unit which is very sensitive to flows near and at zero.